1. Field of the Invention
The present invention relates to a transparent nonvolatile memory thin film transistor (TFT), and more specifically, to a transparent nonvolatile memory TFT in which the physical properties and materials of respective components of a gate stack of the TFT are optimized to allow the TFT to be transparent in a visible light region, undergo a low-temperature process, and perform low-voltage and high-speed operations, and a method of manufacturing the TFT.
2. Discussion of Related Art
The electronics industry has been developing so far through the advancement of silicon-based materials and device technology. Electronic components of the electronics industry are composed of a large number of silicon unit devices. In order to improve performance of the components, methods for integrating a large number of elements into a small area by downscaling the elements are being employed.
Meanwhile, since 2000, the technical development of the electronics industry has taken a somewhat different direction. That is, the electronics industry has led to the coexistence of the above-described conventional silicon-based electronic technology and unprecedented new concepts.
Specifically, new fields that are introducing the new concepts have the following characteristics. First, there is a growing tendency for electronic devices and systems to be formed on flexible substrates, abandoning the conventional silicon-based electronic devices that are hard and breakable. Second, the manufacture of transparent electronic devices and systems is being favored over that of conventional silicon-based electronic devices formed on silicon substrates that are opaque in the visible light region. These two tendencies in the electronics industry are closely related not only to recent fractionalization and diversification of demands among consumers to which conventional devices have not been able to adjust, but also to the successive introduction of integrated applications including multimedia contents and the rapid increase in personal portable devices.
In addition to the demand for high-performance silicon electronic devices, new concepts of electronic devices are being required to satisfy new standards, such as low cost, disposability, portability, and inclinations toward design and health. The first new field implicated therein is flexible electronic devices in which flexible substrates are used, and the second new field is transparent electronic devices having transparent systems. In recent years, techniques in the two new fields have been rapidly developed in the academic and industrial worlds. Also, research is being conducted on developing various applications, such as sensors, displays, electronic circuits, and batteries.
In the above-mentioned field of the transparent electronic devices, the development of transparent thin film transistors (TFTs) and transparent displays using the transparent TFTs as driver circuits is accelerating and entering into a stage of increasing technical readiness and devising target applications to put the devices to practical use. Furthermore, research into techniques of disposing transparent electronic circuits on various substrates using the transparent TFTs is in progress.
Meanwhile, development of techniques related to memory devices for storing data have lagged far behind the brisk development of techniques for displaying and processing data using transparent devices. Of course, since the memory devices (or data storage devices) may be mounted outside systems to perform predetermined functions, there is not as much need for them to be transparent as there is in data display/processing devices. However, if it is possible to mount transparent nonvolatile memory devices with appropriate functions in systems, the systems may have better performance and reduce power consumption and mounting cost. As a result, introduction of systems having new functions can be expected.
A memory device for a transparent electronic system should satisfy the following requirements:
First, the memory device should be nonvolatile. In general, memory devices may be divided into volatile memories and nonvolatile memories. A volatile memory stores data only when power is supplied, while a nonvolatile memory may retain data even when power is interrupted. Since the transparent electronic system is highly likely to be a stand-alone electronic device to which power is always supplied or a design-oriented application with a highlighted mobile function, a nonvolatile data storage function is required to increase the lifespan of a battery and store a large amount of data.
Second, an operating voltage of the memory should be within a predetermined range. When a memory operation requires an excessively high operating voltage through excessive emphasis on transparency, the entire electronic system may be adversely affected, and mounting the memory device in the electronic system or an integrated circuit (IC) becomes unnecessary. In addition, the memory device should ensure stable operations within the range of an operating voltage of a module used along with the memory device.
Third, the memory device should not be excessively large. A transparent memory device for a transparent electronic system would not only store data but also serve as an embedded memory device with additional functions. Accordingly, the memory device should be as small as possible so as not to excessively increase the size of the entire system.
Fourth, the memory device should ensure device stability suitable for operations required by the transparent electronic system. A typical nonvolatile memory device should also satisfy some reliability items.
A first reliability item is a repeated write characteristic, which indicates the number of times a memory device is capable of repeating a write operation. A second reliability item is a memory retention characteristic, which indicates how long the memory device can retain stored data. A third reliability item is an environmental tolerance, which indicates the capability of retaining stored data in hot or humid environments. A transparent memory device for a transparent electronic system would not have to satisfy a high reliability characteristic equal to that of a conventional silicon electronic device, but should satisfy reliability specifications required by the corresponding application.
Although several examples of transparent nonvolatile memory devices have been reported thus far, they perform memory operations based on different principles. Three typical principles on which the reported transparent nonvolatile memory devices operate and merits and demerits of the principles will now be briefly described.
A first method of operating a transparent nonvolatile memory device employs a transparent oxide material having a relatively wide bandgap. In this method, the memory device operates on a principle that the resistance of a transparent oxide thin layer with a wide bandgap varies with application of a voltage. A memory using the first method is typically referred to as an oxide resistive memory, which has been proposed for an advanced nonvolatile memory that will supersede flash memories in the field of conventional silicon electronic devices. In order to apply an oxide resistive memory to a transparent electronic system, all elements constituting the oxide resistive memory must be formed of transparent materials. Accordingly, an oxide thin layer, which is an essential element of the oxide resistive memory, should be formed of a material with a wide bandgap, whose resistance varies within a large range according to the intensity or direction of an applied voltage. The oxide resistive memory device is structurally simple and occupies a relatively small area. However, the principle that the resistance of a material forming the oxide resistive memory device varies according to the intensity or direction of an applied voltage has not yet been closely examined, and it is known that the oxide resistive memory undergoes big differences in characteristics when materials of upper and lower electrodes are varied. In other words, it is difficult to ensure characteristic uniformity of the oxide resistive memory, and the operating principles of the oxide resistive memory are unclear. Thus, the oxide resistive memory cannot be used as an embedded memory for a system.
A second method of operating a transparent nonvolatile memory device involves preparing a charging region in a predetermined portion of a memory device so that a threshold voltage of a transistor can be varied according to the intensity or direction of an applied voltage. The charging region may correspond to a thin layer corresponding a portion of a gate of the transistor or nanodots. A memory using the second method is typically referred to as a nano-floating gate memory, which has also been developed as an advanced flash memory in the field of conventional silicon electronic devices. The manufacture of the nano-floating memory requires a relatively simple, additional process, that is, a process of forming a charging region on a portion of a gate stack of a conventional transparent TFT. However, since the nano-floating gate memory uses an oxide semiconductor thin layer, it is far more difficult to quantitatively control the storage of charges than in a case in which a silicon semiconductor layer is used. Furthermore, the drive of an oxide semiconductor thin layer using an accumulation layer and a depletion layer precludes low-voltage operations.
A third method of operating a transparent nonvolatile memory device employs a ferroelectric thin layer as a gate insulating layer of a TFT so that a threshold voltage of the TFT can be varied according to the direction of an applied voltage using a remnant polarization characteristic of the ferroelectric thin layer. A memory using the third method is typically referred to as a transistor-type ferroelectric memory, which has also been developed as an advanced nonvolatile memory in the field of conventional silicon electronic devices. Similarly, the manufacture of the transistor-type ferroelectric memory requires a relatively simple process, that is, a process of forming a ferroelectric thin layer instead of a gate insulating layer without changing the structure of a conventional transparent TFT. Also, physically predictable, exact operating principles based on the remnant polarization of a ferroelectric material may be advantageous to designing the transistor-type ferroelectric memory device. However, when the ferroelectric thin layer is formed of an oxide material, the oxide material needs to be crystallized so that its ferroelectric characteristics can be used for device operations. Since an oxide-based ferroelectric thin layer is typically crystallized at a temperature of about 500° C. or higher, the crystallization is not compatible with a process of forming a transparent oxide semiconductor layer at a temperature of about 300° C. or lower. In addition, when a ferroelectric thin layer is formed of an organic material, a large leakage current is generated and it is difficult to thin out the ferroelectric thin layer and apply the ferroelectric thin layer to memory devices.